The measurement of particulates carried along in a gas stream, e.g. in a smoke stack, is sometimes difficult. Interference with or diminution in intensity of a light beam is one method which is commonly practiced. Generally speaking, such a system includes a light source, a photodetector and means to calibrate lowering of the output of the photodetector with the particulate burden of the gas flowing between the source and the detector. Some difficulties may be encountered with such a system. Firstly, at best, the system will measure particulate concentration only in the path of the light beam which usually is only a minute fraction of the cross-sectional area of the gas duct or flue carrying the particulate. Secondly, surfaces on both the light source and the detector are likely to get dirty rather quickly leading to error in calibration of the system and the need for frequent cleaning. As a third point, condensate in a chimney or flue such as microscopic and submicroscopic water or sulfuric acid particles can scatter the light beam and thus indicate a particulate load, i.e. a solid particulate burden greater than what actually exists. As an additional point, a light beam detector may be overwhelmed by the amount and/or the color of levitated particles and, once the gas stream becomes opaque to the light, no additional particulate can be detected. Finally, especially in stacks, flues and chimneys carrying off-gases from sulfide oxidation, light beam source and detector parts are subject to significant corrosion in use.
Another system often used for estimation of the particulate burden of a gas flow comprises essentially particulate entrapment by filtering, impactment and capture or the like. This type of system can be very accurate but usually cannot be operated in a sufficiently timely fashion so as to provide data useful for control of a collection device such as a Cottrell.TM. precipitator.